1. Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for early soft handoff in a code-division multiple access (CDMA) communication system.
2. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A, IS-95B and IS-95C (also known as CDMA2000) entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.xe2x80x9d Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled xe2x80x9cPersonal Stationxe2x80x94Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems.xe2x80x9d Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in U.S. patent application Ser. No. 5,805,648, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
The above standards and patents describe, among other things, the manner in which a mobile station is to execute a xe2x80x9chandoffxe2x80x9d between neighboring base stations as it travels between their respective geographic coverage areas. For example, in the CDMA-based standards IS-95 and J-STD-008, the base station sends a message to the mobile station listing many of the system parameters of its neighboring base stations, including such information as would assist the mobile station in executing an xe2x80x9cautonomousxe2x80x9d handoff between base stations. An autonomous handoff is one that is not initiated or directed by the base station, but rather is initiated by the mobile station itself.
An example of one such neighbor list message is the xe2x80x9cExtended Neighbor List Messagexe2x80x9d of J-STD-008. When the base station sends an Extended Neighbor List Message to the mobile station, it uses the format of Table I.
The above table is taken from Section 3.7.2.3.2.14 of J-STD-008, and indicates the various fields transmitted in an exemplary Extended Neighbor List Message. Of particular concern to the present invention are the following fields:
NGHBR_PNxe2x80x94the base station sets this field to the pilot PN sequence offset for this neighbor, in units of 64 PN chips; and
NGHBR_FREQxe2x80x94the base station sets this field to the CDMA channel number corresponding to the CDMA frequency assignment for the CDMA channel containing the paging channel that the mobile station is to search.
Thus, according to J-STD-008, the mobile station is given the frequency and PN offset of each neighboring base station. This gives the mobile station enough information to make a more focused search for neighbor pilots, rather than having to search all possible PN offsets on all possible CDMA frequency assignments. For example, the mobile station may keep a table of all the neighbors that were passed to it in the neighbor list message or extended neighbor list message. Such a table might resemble Table II below.
With respect to xe2x80x9csame-frequency neighbors,xe2x80x9d i.e. those that are on frequency f(1), the very nature of a CDMA modulation scheme allows a mobile station with a diversity receiver such as that described in the above-mentioned U.S. Pat. No. 5,109,390 to search for other pilot signals on the same frequency assignment, but having different PN offsets, while simultaneously continuing to demodulate any channel that it is already monitoring. In other words, a CDMA mobile station is typically able to search for the pilot signals of other base stations on the same frequency assignment, without interrupting transmission or reception of data with its original base station.
Current Cellular/PCS systems do not have the capability to handle large data transfers that occur in computing situations including internet environments. What is needed is a solution to carry large amounts of data in addition to voice.
An embodiment of the invention provides a method and apparatus for high data rate communication in a cellular/PCS communication system. Specifically, the embodiment provides for early assignment of code channel for signaling channels (F-DCCH) in soft handoff. Thus, rather than simultaneously assigning a code channel for signaling data and another for traffic data for a particular Mobile Station (MS) in soft handoff, the code channel is assigned earlier for signaling data. Some of the unnecessary Base Station (BS) procedures with respect to the signaling data may be eliminated. An embodiment includes the concept of early signaling from the MS to request a code channel for signaling data, optionally followed by signaling (at a later time) from the MS to request a code channel for traffic data. This permits the MS to have the F-DCCH in soft handoff before having the F-DTCH (Forward Dedicated Traffic Channel) in soft handoff without requiring the BS to skip any of its procedures.